Buttercream To Soft

Hi I'm a little nervous, as I have to make a cake for a colleague on Wed (baking wed, decorating thurs, taking to work on Fri).

HOWEVER! My buttercream lately is coming out too soft and I worry it wont last til Sat when she needs it. I'm using the All American Buttercream cos we dont have shortening in SA. Do you think I'm doing anything wrong?

I'd appreciate any tips to help my buttercream firm up (I made cupcakes this weekend, and put buttercream in fidge overnight to firm up, but while spreading it on the cupcakes this morning, it was already getting soft :S)

That recipe looks to be 2 ounces short on powder4ed sugar. The rule of thumb for that style of bc is 1 cup (8 oz) of fat per pound of ps. Since it calls for 12 oz butter, you'll need 1.5 (1 pound, 8 oz) pounds of ps.